Hundreds of websites across Europe offering "free" or "cheap" mobile phone services such as ringtones and wallpaper are conning young people into taking out expensive contracts, the European commission warned yesterday.

The websites, including 39 in Britain, face enforcement action, including substantial fines and closure, after being suspected of breaching EU consumer rules, the EC said.

"Far too many people are falling victim to costly surprises from mysterious charges, fees and ringtone subscriptions they learn about for the first time when they see their mobile bill," said Meglena Kuneva, EU consumer commissioner.

She was giving the results of a unique EU-wide "sweep" last month by national regulators of 558 websites in the 27-nation bloc and Norway and Iceland.

Around 80% or 446 - half of which target young people with devices such as cartoon or TV characters - are to be further investigated. In the UK, 39 out of 43 sites covered by the sweep are suspected of giving misleading information.

Kuneva's warning of EU-wide enforcement action to track down each of the suspected traders comes just days after the EU telecoms commissioner, Viviane Reding, proposed wide-ranging cuts in the cost of sending mobile text messages when travelling within Europe.

Their actions, plus measures to make it easier for customers to switch bank accounts and enjoy lower charges, are part of an EC drive to make the EU more popular as a consumer champion when it has fallen out of favour with its 500m citizens.

Ringtones make up 29% of the European "mobile content" market and, growing at 10% a year, saw sales rise to €691m (£548m) in 2007. More than 495m mobile phones are owned by Europeans.

PhonepayPlus, the UK regulator, said the UK market for mobile content was worth £460m and complaints rose to 4,500 in the first three months of this year.

Its chief executive, George Kidd, told the BBC: "There is a clear lack of trust among many consumers about mobile premium services and this is small wonder when you consider the kind of harm that is being done to them by some providers."

The EU sweep found almost half the websites offered missing or hidden information about the offer's price, with the customer only finding out via their bill. More than 70% lacked basic information on how to contact the trader and more than 60% had misleading information, including widespread use of the word "free". Kuneva cited the case of a consumer's nine-year-old daughter who ordered a "free" ringtone but found this led to expensive membership. The operator paid back €125 in unwanted messages but has retained €273.